Electrically switched DC motors, such as stepper motors or more generally brushless motors, are used in numerous control and regulation applications and also in systems for driving mass memory devices such as hard disks, floppy disks, optical disks, CD-ROMs, etc.
Hereinafter reference will be made to a three-phase electric motor, though the same observations also hold for a generic poly-phase motor.
Brushless motors may be driven using an integrated circuit, commonly called “smooth driver”, of the type shown in FIG. 1 and illustrated in the document US 2009/0026990 herein incorporated by reference in its entirety. The output stage is represented by a full-wave bridge circuit that, in the case of a three-phase motor, uses six bipolar (BJT) or field effect (MOS) power transistors, as shown in the figure.
Typically, a brushless motor is driven by properly supplying the phases of the motor synchronously with the instantaneous position of the motor. This may be done by energizing sequentially two windings of the motor with positive and negative voltages, respectively, leaving a third winding in a high impedance state. When a brushless “sensorless” motor is driven, the not energized winding is exploited for sensing the position of the rotor. The driving voltages or currents applied to the motor windings, instead of having a pre-established constant level during each switching phase, have a certain digitized pre-established (non constant) driving voltage or current waveform stored in a nonvolatile static memory device, for example, in an EPROM or EEPROM memory.
This technique is well known in the art, for example from the European patents EP 800262, EP 800263, EP 809349 and from U.S. Pat. No. 6,137,253 the disclosures of which are herein incorporated by reference in their entireties, and for this reason will not be illustrated further. A basic scheme of the driver is depicted in FIG. 2, in which the functional blocks and the indicated signals have the meaning listed in the following table:
Profile AMemory device storing the voltagewaveform for winding AProfile BMemory device storing the voltagewaveform for winding BProfile CMemory device storing the voltagewaveform for winding CPower AHalf-bridge for driving winding APower BHalf-bridge for driving winding BPower CHalf-bridge for driving winding CMotorMotorPositionPosition signal of the rotorControllerControl circuitSpeed MeasureCircuit for measuring the rotorspeed starting from the signal ofthe rotor positionSpeedRotor speedSpeed ControlControl circuit of the rotor speedTorque ControlMotor torque optimization circuitKVALSignal proportional to a desiredvalue of motor torqueAddr. GeneratorGenerator of memory addresses forreading waveformsTorque OptimPhase angle (start of a readingfrom the memory) in respect to theBEMF of a voltage Vin applied to awindingReset addressStart signal for reading awaveform from the memory forobtaining a synchronous waveformwith the rotor positionProfile OutVoltage waveform read from thememoryVinControl voltagePhA currentPhase current in winding A
In order to properly supply the windings, the position and speed of the rotor of the motor are determined with a feedback circuit for sensing or estimating the back electromotive force (BEMF) induced in the tristated winding.
The driver allows a voltage mode driving typically used for permanent magnet synchronous motors PMSM because it is simpler than current mode driving. The values KVAL and Torque Optim are provided as inputs; and, as a function thereof, a phase voltage Vin to be applied at the motor windings is generated. In order to make the applied voltage independent from eventual fluctuations of the supply voltage of the motor, closed-loop compensation methods may be used as well as open-loop systems with feed-forward compensation. An example of feed-forward compensation is described in the U.S. Pat. No. 6,150,963 in the name of the same applicant, herein incorporated by reference in its entirety.
Considering purely sinusoidal or pseudo-sinusoidal (i.e. step waveforms that approximate sinusoidal waveforms) phase voltages, the driving of each winding of the electric motor is fully described by the phasors of FIGS. 3, 4 and 5, the labels having the following meaning:
TorquePhase angle (start of a reading from theOptimmemory) in respect to the BEMF of a voltageVin applied to a windingKVALSignal proportional to a desired value ofmotor torqueBEMFBack electromotive force induced by therotation of the motorRResistance of the windingLInductance of the windingIPhase current flowing throughout the windingVinControl voltage of the windingVdiffResistive-inductive voltage drop on thewindingγPhase angle between phase current and backelectromotive forceBPhase angle between the resistive-inductivevoltage drop and the induced backelectromotive forceAPhase angle between the resistive-inductivevoltage drop and the motor coil currentMotorMotor windingphaseΩAngular speed of the motor
As clearly shown by the phasors of FIGS. 3, 4 and 5, in a voltage mode controlled motor the current flowing throughout the windings of the motor is not directly controlled and depends, besides upon the input driving signals that fix the values of KVAL and Torque Optim, also upon other factors such as the load characteristics (resistance and inductance of the stator windings) and upon the value of the back electromotive force BEMF generated by the motor itself, as well as upon the phase angle between the applied voltage and the BEMF of the phases. The torque generated by the motor is not solely determined by the value of the input signal as in current mode drivers.
Typically, the value KVAL is fixed as a function of the desired motor speed, and the value Torque Optim is fixed accordingly to optimize the motor torque once the desired value of speed is established.
Being that the motor torque of a PMSM motor is a function of the phase angle between the stator phase current and the related BEMF induced at the same phase, it may be important in voltage mode systems to have the possibility of controlling the stator current i with respect to the BEMF (on its rotational function of the rotor position), besides its amplitude.
In the known voltage mode driving of FIG. 2, the phase of the voltage Vdiff with respect to the back electromotive force BEMF is a nonlinear function of the amplitude of the voltage Vin, of the amplitude of the BEMF and of the phase angle between the voltages Vin and BEMF. It is relatively easy in steady state conditions, that is, with constant rotation speed and resistive torque, to establish the phase of the voltage Vin with respect to the BEMF for forcing a current in phase with the back electromotive force BEMF, but this is relatively complicated (or realizable with unsatisfactory pass band performances) in non steady state conditions (for example, during torque transient conditions).
FIG. 3a is a simplified electric diagram that illustrates a voltage mode driving implemented with the known device of FIG. 2 wherein a sinusoidal driving voltage Vin in phase with the BEMF (Torque Optim=0) is applied, thus also the resistive-inductive voltage drop Vdiff is in phase with the voltages Vin and BEMF, as shown in the phasor diagram of FIG. 3b. The phase current i through the winding of the motor is out of phase with respect to the voltage Vdiff (and thus with respect to the BEMF in this case) by an angle α=arctan(ωL/R) that may be relatively great. The phase angle between the phase current i and the BEMF may be very important for defining the motor torque and the efficiency of the PMSM motor driving, thus it is clear that it is useful to control the phase angle of the voltage Vin with respect to the phase of the BEMF (that is Torque Optim) in order to drive indirectly the phase angle of the voltage Vdiff and thus the phase angle of the current. In other words, by acting on an easily controllable parameter (phase angle of the voltage Vin with respect to the BEMF), a variation of the phase angle between the phase current i and the BEMF is indirectly obtained.
A drawback of this driving technique is that there is not solely a relationship between the phase angle between the voltage Vin and the BEMF and the phase angle between the BEMF and the phase current i. An optimal phase angle value (Torque Optim “optimal”) could be obtained, for example, through the calibration procedure or through appropriate calculation carried out by a microcontroller or through look-up table or other method. For ease it is common practice to use a constant phase angle value Torque Optim during the normal functioning of the motor: this ensures good performance in most functioning conditions, in which the motor is in steady state conditions, i.e. with a constant rotation speed and resistive torque.
FIGS. 4a and 4b are similar to FIGS. 3a and 3b and contemplate the case in which the control voltage Vin leads the BEMF (Torque Optimizer=K >0), thus the current i flowing throughout the winding will be in phase with the BEMF (maximum efficiency). In steady-state conditions it is possible to identify a value of constant phase angle Torque Optim at which the maximum efficiency is attained, but it is not so during a transient variation of the load.
As described by the phasors of FIGS. 5a, 5b and 5c respectively before a transient, during a transient that requires an increase of the motor torque and during a transient that requires a reduction of the motor torque, the circuit SPEED CONTROL for controlling the speed may cause a sudden variation of the signal KVAL, causing a variation of the current amplitude (desired effect) and a variation of the phase angle of the current i with respect to the BEMF (undesired effect).
In particular, FIG. 5b shows that, after an increase of the requested motor torque: the control voltage increases (Vin′>Vin); the BEMF has remained almost unchanged with respect to the situation shown in FIG. 5a, because the speed ω of the motor cannot vary instantaneously because of inertia; the phase angle α=arctan(ωL/R) between the current i and the resistive-inductive voltage drop Vdiff has remained almost unchanged; the value of the phase angle Torque Optim has remained unchanged; and thus there is an increase of the amplitude of the current i but also a variation (delay) of its phase in respect to the BEMF (γ<0).
Similarly, FIG. 5c shows that, because of a reduction of the requested motor torque: the control voltage decreases (Vin″<Vin); the BEMF has remained almost unchanged with respect to the situation shown in FIG. 5a, for the above reasons; the phase angle α=arctan(ωL/R) has remained almost unchanged for the same reasons; the phase angle Torque Optim has remained unchanged; and thus there is a reduction of the module of the current i but also a (positive) variation of its phase angle in respect to the BEMF (γ>0).
In transient conditions, the PMSM motor is not driven in conditions of maximum efficiency.
Having a current flowing throughout the windings of the motor that varies in phase depending on the requested motor torque (KVAL), may be considered, in certain cases, an undesirable limitation.